Milky biaxially oriented polyester films are known from the prior art. These known prior art films have good optical properties, but are difficult to produce.
DE-A 2 353 347 describes a process for producing milky polyester film having one or more layers, which comprises preparing a mixture from particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film and biaxially orienting the film by stretching in directions running perpendicular to one another, and heat-setting the film. A disadvantage of this process is that regrind which arises during production of the film (essentially a mixture of polyester raw material and ethylene copolymer or propylene copolymer) cannot be reused without yellowing the film. This makes the process uneconomic, but the film produced with regrind would not gain acceptance in the market. If the amount of copolymer in the polyester is increased, the film loses its milky character and turns white with high opacity.
The object of the present invention was to provide a milkily translucent, biaxially oriented polyester film which has improved ease of production, i.e. low production cost. In particular, it should be possible for the cut material (regrind) directly associated with the production process, in an amount of from 10 to 70% by weight based on the total weight of the film, to be reused for the production process without any significant adverse effect on the physical or optical properties of the film produced using regrind. In particular, the addition of regrind should not cause any significant yellow coloration in the film.